We were thinking of a moving target as well. Perhaps something that moves on a rail system. But I like the idea of an old printer carriage.
Its a bit small but the idea works.
I've since ordered a larger toothed belt and some other bits and hope to get on with it soon. As well as a moving target, I like the idea of the target being able to appear at will, i.e. drop out of sight, move, then pop up. I now actually prefer the idea of a paper target but the problem there is counting the score which is something I'm sort of looking into but is proving to be difficult if not impossible to do without a serious amount of ££ which isnt an option for a home project ! Perhaps making the target tray part interchangable from a duck/target/whatever might be the best option. I'll post an update as and when, hopefully later this week.